The present invention relates to epoxidation catalysts, in particular catalysts based on titanium zeolite. It also relates to the use of these catalysts in epoxidation reactions, as well as to epoxidation processes in the presence of these catalysts.
It is known to use catalysts based on titanium silicalite in epoxidation reactions. For example, in patent application EP-A2-0 200 260, microspheres based on titanium silicalite which have a diameter of about 20 xcexcm and are obtained by atomization, are used in epoxidation reactions. This known catalyst gives rise to a deactivation phenomenon. Regeneration cycles, requiring handling operations, are therefore necessary. When these catalysts, with a relatively small diameter, are used in epoxidation reactions it is difficult to isolate them from the reaction medium so that they can be transferred to a regeneration treatment.
The object of the present invention is to resolve this problem by providing a novel catalyst which is easy to separate from the epoxidation reaction medium with a view to sending it to a regeneration unit. A further object of the invention is to provide an epoxidation catalyst which has good mechanical strength, high catalytic activity and high selectivity.
Yet another object of the invention is to provide a catalyst which is easy to use in a fixed or fluidized bed.
The present invention therefore relates to an epoxidation catalyst based on titanium zeolite, which is in the form of extruded granules. It has been found that such a catalyst has the following combined advantages:
it is easy to separate from the epoxidation reaction medium with a view to sending it to a regeneration unit,
it has good mechanical strength, high catalytic activity and high selectivity,
it is easy to use in a fixed or fluidized bed.
The term titanium zeolite is intended to mean a solid containing silica which has a microporous crystalline structure of the zeolite type and in which a plurality of silicon atoms are replaced by titanium atoms.
The titanium zeolite advantageously has a crystalline structure of the ZSM-5, ZSM-11 or MCM-41 type. It may also have a crystalline structure of the aluminium-free xcex2 zeolite type. It preferably has an infrared absorption band at about 950-960 cmxe2x88x921. Titanium zeolites of the silicalite type are highly suitable, Those satisfying the formula xTiO2(1xe2x88x92x)SiO2 in which x is from 0 0001 to 0.5, preferably from 0.001 to 0.05, give good performance. Materials of this type, known by the name TS-1, have a microporous crystalline zeolite structure similar to that of the ZSM-5 zeolite. The properties and main applications of these compounds are known (B. Notari; Structure-Activity and Selectivity Relationship in Heterogeneous Catalysis; R. K. Grasselli and A. W. Sleight Editors; Elsevier; 1991; p. 243-256). Their synthesis has been studied, in particular, by A. Van der Poel and J. Van Hooff (Applied Catalysis A; 1992; Volume 92, pages 93-111). Other materials of this type have a structure similar to that of beta zeolite or ZSM-11 zeolite.
The term extruded granules is intended to mean grains obtained by extrusion. In particular, the granules are obtained by extruding an extrudable mass containing the titanium zeolite and by cutting the extrudate emerging from the extruder into grains.
The shape of the extruded granules is arbitrary. They may be solid or hollow They may be of round or rectangular cross-section, or alternatively a different cross-section with a greater external surface area. Cylindrical shapes are preferred. When they are of cylindrical shape, the extruded granules advantageously have a diameter of at least 0,5 mm, preferably of at least 1 mm. The diameter is usually at most 5 mm, particularly at most 2 mm. The cylindrical shapes have usually a length of at least 1 mm, particularly of at least 2 mm. Lengths of at most 8 mm are current, those of at most 4 mm give good results. The cylindrical shapes having a diameter of from 0.5 to 5 mm, preferably from 1 to 2 mm, and a length of from 1 to 8 mm, preferably from 2 to 4 mm are suitable.
The content of titanium zeolite in the catalyst according to the invention is generally at least 1% by weight, in particular at least 50% by weight. The content of titanium zeolite is most often at most 99% by weight, particularly at most 98% by weight. The catalyst according to the invention generally contains from 1 to 99% by weight, preferably from 50 to 98% by weight, of titanium zeolite, the remainder consisting of a matrix. This matrix preferably contains a siliceous material.
The catalyst according to the invention can be obtained by a process comprising:
(a) a step of blending a mixture comprising a titanium zeolite powder, water, at least one binder, at least one plasticizer and optionally other additives, in order to form a paste,
(b) a step of shaping the paste obtained in step (a) by extrusion, in order to obtain an extrudate,
(c) a step of drying in order to remove at least some of the water,
(d) a step of calcining in order to remove at least some of the organic residues present, and comprising a granulation step carried out between the extrusion step (b) and the drying step (c) or after the calcining step (d), in order to obtain extruded granules.
Step (a) generally consists in mixing a titanium zeolite powder with water, at least one binder, at least one plasticizer and optionally other additives until a paste is obtained with a viscosity such that it can be employed in an extruder. The mixing may be carried out in any kind of mixer or blender. All the constituents of the mixture can be mixed simultaneously. As a variant, the binder, the plasticizer, the water and, if appropriate, the other additives may be premixed before they are added to the titanium zeolite powder. The mixing is advantageously carried out at room temperature. As a variant, the mixture may be cooled during step (a), for example with water. The duration of step (a) can vary from 5 to 60 minutes.
The particle size distribution of the titanium zeolite powder employed in step (a) can vary greatly. It is preferably characterized by a mean diameter of less than or equal to 10 xcexcm, in particular less than or equal to 5 xcexcm. The mean diameter is generally at least 0.05 xcexcm, in particular at least 0.1 xcexcm. Diameters of less than 0.05 xcexcm are also suitable.
The plasticizer which can be used in step (a) may be a polysaccharide such as a starch or a cellulose. Celluloses are highly suitable. By way of examples of cellulose, mention may be made of methyl cellulose, carboxymethyl cellulose and hydroxyethyl cellulose. Methyl cellulose is preferred.
The amount of placticizer employed in step (a) can vary greatly. Small amounts of at least 1% and less than 10% by weight relative to the weight of titanium zeolite employed are recommended because they lead to better resistance to attrition in comparison with higher amounts.
The binder which can be used in step (a) may be selected from silicon derivatives such as siloxanes. By way of examples, mention may be made of methyl siloxane or ethyl siloxane ethers. Silicone resins based on polymethylsiloxane may also be used. Silicone resins of the polymethyl/phenylsiloxane type are also suitable. It is also possible to use mixtures of different oligomers of the methylsiloxane type. The binder employed in step (a) may be in the form of a powder As a variant, it may be in the form of an aqueous emulsion. It may also be used in liquid form Silicone resins based on polymethylsiloxane in the form of a powder, and mixtures of different oligomers of the methylsiloxane type in liquid form are preferred because they lead to catalysts with higher mechanical strength. In the calcining step (d), the binder is converted into a material forming the matrix which is present in the catalyst according to the invention.
The amount of binder employed in step (a) can vary greatly. It is customarily at least 3% by weight, in particular at least 5% by weight, relative to the weight of titanium zeolite employed. It is commonly at most 70% by weight, in particular at most 30% by weight, relative to the weight of titanium zeolite employed. Amounts of from 5 to 20% by weight relative to the weight of titanium zeolite employed are particularly highly suitable because they lead to a better compromise between the catalytic activity and the mechanical strength in comparison with lower or higher amounts.
Lubricators may also be added to the mixture of step (a). These may be compounds based on paraffin, polyvinylpyrrolidone, polyethylene oxide and polyvinyl alcohol.
Pore-forming substances may also be added to the mixture of step (a). These substances are removed during the calcining step (d) and thus increase the porosity of the catalyst. Melamine may be mentioned by way of an example of a poreforming substance. The amount of pore-forming substance employed is generally at least 5% by weight, in particular at least 6% by weight, relative to the weight of titanium zeolite employed. It is customarily at most 35% by weight, in particular at most 14% by weight, relative to the weight of titanium zeolite employed. Amounts of from 6 to 14% by weight relative to the weight of titanium zeolite employed are particularly suitable because they lead to better resistance to attrition in comparison with higher amounts.
The extrusion step (b) may be carried out in a piston extruder. As a variant, it may be carried out in a screw extruder.
The drying step (c) is advantageously carried out at low drying rates in order to ensure that the catalyst has a high degree of cohesion. For example, predrying at low temperature (for example from room temperature to 90xc2x0 C., optionally in combination with infrared or microwave irradiation) may be carried out first, the temperature may then be raised gradually in order to reach the final drying temperature As a variant, when water can be evacuated rapidly by suitable ventilation, the temperature may be increased at a higher rate. The temperature is typically raised at a rate of 1xc2x0 per minute. The drying is generally carried out at a final temperature of at least 400xc2x0 C. The final drying temperature is customarily at most 500xc2x0 C. Lower temperatures from 100 to 400xc2x0 C. may be suitable when the drying time is long enough, for example 10 to 20 h.
The calcining step (d) is generally carried out at a temperature of at least 300xc2x0 C., in particular at least 400xc2x0 C. The temperature is customarily at most 550xc2x0 C., in particular at most 520xc2x0 C. Temperatures in excess of 550xc2x0 C. are not recommended because most titanium zeolites cannot withstand such temperatures. The duration of the calcining step (d) must be long enough to make it possible to remove most of the organic residues originating from the binder and/or the plasticizer. Durations of 60 h are typical. Generally, the duration is at least 50 h and at most 100 h. The calcining step (d) is preferably carried out in an oxidizing atmosphere, preferably in air.
As it is described above, the process comprising steps (a) to (d) and a granulation step can be used to prepare other catalysts in the form of extruded granules.
The catalyst according to the invention can be used in the synthesis of epoxides by reacting an olefinic compound with a peroxide compound.
The invention therefore also relates to the use of the catalyst described above in these syntheses.
The invention also relates to a process for the preparation of an epoxide by reacting an olefinic compound with a peroxide compound in the presence of the catalyst described above. The epoxide is preferably 1,2-epoxy-3-chloropropane or 1,2-epoxypropane. The olefinic compound is preferably allyl chloride or propylene. The peroxide compound can be chosen from those containing active oxygen and capable of carrying out epoxidations. Hydrogen peroxide and those peroxide compounds which can produce hydrogen peroxide under epoxidation reaction conditions are suitable. The peroxide compound is preferably hydrogen peroxide.